Electronic organ



1951 P. J. GREAVES ET AL ELECTRONIC ORGAN 2 SheetsfiSheet 1 Filed Dec. 2, 1947 /NVN7'OA PUL FORD r1. 6mm vss FEEDER CA N. [AGAWR A fro/Mir Patented Oct. 2, 1951 ELECTRONIC ORGAN Pulford J. GI-eaves, Dearborn, and Frederick N. Elgear, Detroit, Mich.

Application December 2, 1947, Serial No. 789,250

7 Claims. I

This invention relates to devices for the electrical reproduction of musical sounds and particularly to an electrical instrument of the organ type.

In the light of past developments in this field, an electrical musical instrument may be considered to be composed essentially of a generator of electrical frequencies corresponding to the acoustical frequencies of the tones to be reproduced, amplifying means for transforming these electrical frequencies into sound, and keying means to control the passage of the electrical frequencies from the generator to the amplifier according to the will of the player. To basic equipment for the accomplishment of these functions have been added devices for controlling the volume of sound, the timbre of the tone and various other accessories contributing to a more complete simulation of the performance of conventional instruments.

Electrical musical instruments incorporating the foregoing elements have been in commercial production for some time during which various improvements and refinements have been made so that such instruments now occupy a recognized position in the field of music. This is particularly true with respect to the electric organ type of instrument. However, the present electric organ is an extremely intricate and costly instrument and is subject to definite limitations in the degree of musical expression that can be obtained even by the most accomplished players. It is the general object of the present invention to simplify the construction of instruments of this type while also providing an instrument where the accomplished musician can attain complete musical expression of all notes played.

This general object of the invention may be broken down into the more specific objects which follow:

To provide improved and simplified means of generating electromotive forces having pulsating frequencies corresponding to fundamental frequencies of the tempered music scale.

To provide a frequency generator assembly in which a plurality of electro-motive forces are produced, each having a pulsating frequency of musical significance.

To provide a series of such frequency generator assemblie arranged so that each succeeding assembly produces frequencies of a higher harmonic.

To provide means for independently varying the strength of the electro-motive forces produced by each generator assembly.

To provide means for selectively combining the fundamental frequency of any particular musical tone with the harmonics of that tone so that the resultant complex frequency can be transformed into sound under the control of a key.

To provide means for regulating the relative strength of the fundamental and all harmonics thereof so that a great variety of complex fre quencies may be produced.

To provide a new key construction which permits each note on the keyboard to be individually and completely expressed according to the type of movement given to the key; which permits the electrical frequency of each note to be directed to an amplifying system without the use of electrical contacts; which eliminates key click and other objectionable noise from the keyboard; which permits the strength of the electrical frequency delivered to the amplifying system to be proportional to movement of the key; which eliminates the problem of aftertone following the release of a key; and which permits any desired number of pure frequencies to be combined to form a complex frequency directly at the key, thereby simplifying the problem of bringing a fundamental frequency and a number of harmonics thereof under the control of a single key.

To provide novel means for varying the overall volume of the keyboard in the manner of a conventional organ swell pedal.

To provide, in combination with frequency generators and keys, means for transforming the electrical frequencies produced and controlled thereby into sound.

With the foregoing more important objects in mind the construction and manner of operation of the invention will be described in steps corresponding to the major functional elements of instruments of this type, i. e. the frequency generator, the key construction and the association of these elements with an. amplifying system. This description is augmented by the accompanying drawings which form a part of this specification and consist of the following views:

Fig. 1, a schematic view showing a simplified electrical circuit of an organ and the relation" ship between a series .of frequency generators, a single key of a keyboard and an amplifying circuit.

Fig. 2, an elevation of a frequency generator with a portion of the side thereof broken away.

Fig. 3, a sectional view of this frequency generator taken along the line 33 of Fig. 2.

Fig. 4, a schematic wiring diagram of the elements for the generation of a single frequency.

Fig. 5, an elevation showing the key inechanism of the invention.

Fig. 6, a similar view of an alternate type of key construction.

Fig. 7, a swell pedal or volume control shown in elevation.

The frequency generator Probably the simplest and most widely used device to produce musical tones is the vibrating reed, and furthermore, it has been recognized that such a reed acting as one plate of a condenser could, when vibrating, produce a pulsating voltage of the same frequency as the vibration on an adjacent electrode acting as the second plate of a condenser. However, to our knowledge no adaptation of the vibrating reed as a frequency generator for an electrical organ has been made. There are at least two main reasons for this, the first having to do with the nature of the frequency produced by a vibrating reed and the second with the control of the electric frequency generated between the reed and the amplifier.

In an organ it is most desirable that each frequency generated have the smooth wave characteristic of a pure tone so that the frequency of any tone used as a fundamental can be blended in exact proportion with the frequencies of any of the harmonics of that tone under the control of the player. A vibrating reed does not naturally produce a pure tone but instead gives off a complex vibration which includes certain harmonics in addition to the fundamental. In other words, the natural sound wave of a vibrating reed is complex in contradistinction to the pure wave desired.

Secondly, in any type of electrical musical instrument when a vibrating reed is used as one plate of a condenser, the electro-motive force produced on an adjacent electrode is electrostatically generated, and it has been found that when electrostatically generated frequencies are used with known, commercially feasible types of key-operated devices for controlling the passage of such frequencies from the generator to the amplifier, a problem of aftertone is introduced. The sound continues to come from the instrument for an appreciable interval of time after the key has been released. Since the objections to aftertone are obvious, electrostatic methods of generating frequencies have been generally discarded because of the lack of a simple means of accurately regulating their translation into sound. Our invention includes the provision of a new type of keying device which eliminates this objection to electrostatic generation and which will be separately described under the heading Key construction later in this specification.

The frequency generator of the present invention employs a vibrating reed for the production from the vibration thereof of a pulsating potential rather than sound. Fig. 4 schematically shows the electrical circuit employed for single reed H which is connected to a source of potential through a potentiometer l2. An insulated electrode 13 is located adjacent the reed so that when a voltage is impressed upon the circuit, the reed acts as one plate of a condenser with the electrode [3 forming the other plate. If the reed I! is made to vibrate, the air gap between the reed and the electrode l3 will vary causing a row of reeds.

each row.

- that frequency.

corresponding fluctuation in capacitance and producing a similar variation in the charge induced on the electrode 13. We have found that by making the eletcrode with a very small area and by positioning this electrode either near the end of the reed or across from the end of the reed and along the neutral longitudinal axes as shown, the voltage variation on the electrode will follow the desired smooth wave form with little or no harmonic content. The optimum position of the electrode relative to the reed for minimum amount of harmonic content may not be exactly the same for all lengths of reeds, but, if after locating the electrode in this optimum position any harmonic content remains in the wave of electrode voltage variation, such harmonic content can be removed by a suitable filter 14.

The conventional organ has sixtyone keys covering a range of five octaves of the tempered music scale beginning with the note C having a frequency of 32.70 cycles per second to the fifth octave C with a frequency of 1046.50 cycles per second. Accordingly, a complete frequency generator assembly, such as is shown in Figs. 2 and 3, will consist of sixty-one reeds I5, each having a natural period of vibration corresponding to the fundamental frequency of each of these sixty-one notes. These sixty-one reeds are mounted in two rows within a case [6, each row of reeds running lengthwise of the case along a support member I! and a bus bar I8 to which each reed is attached. A slotted baffle plate l9 extends downwardly from the top of the case along each row of reeds so as to lie adjacent to the top and sides of each reed in the row. At the bottom of the case between the rows of reeds a substantially fan-shaped air inlet duct 20 is provided, the outer sides 2| of this duct being formed as continuations of the support members 11. An air intake pipe or hose connection 22 is located centrally of the air duct 20. Two air outlet ducts 23 similar in construction to the air inlet duct are provided, one on each side of the case and extending along the length of a Air supplied to the case enters through the inlet duct to a chamber 24 formed between the battle plates I9 and in passing from this chamber to either of the outlet ducts 23 the flow of air causes a vibration of all the reeds in An electrode 25 is furnished for each one of the reeds, these electrodes being mounted so as to project inwardly from the side of the case to a point adjacent the end of each reed. Each electrode is mounted so as to be insulated from the case and so that the distance between the end of an electrode and the reed with which it is associated can be adjusted. This mounting consists of a threaded boss 26 formed with or attached to the side of the case and both the boss and side of the case are drilled to receive a sleeve 21 of suitable insulating material. The electrode 25 extends through this sleeve and is attached to a cap 28 of Bakelite or other non-conducting material having internal threads to engage the threaded boss.

Electrically the bus bars l8 and hence each of the reeds I5 are connected in parallel to a source of potential through a circuit similar to that shown in Fig. 4 and including a potentiometer for controlling the strength of the voltage impressed upon the reeds. The pulsating potential picked up by each of the electrodes upon vibra tion of the charged reeds is led to the particular key assembly corresponding to the note having Since it is practically impossible to construct a frequency generator of this type wherein the flow of air would be so dis tributed that all the reeds would vibrate with the same amplitude of movement and since it is desirable that the potential inducted at each electrode be of approximately equal magnitude, the electrodes are each mounted so as to be adjustable to and from the reeds in the manner previously described. Thus, if any particular reed vibrates with less amplitude than others, its electrode can be brought closer increasing the induced charge by simply turning the cap 28, and conversely, the induced charge on an electrode adjacent a reed having a large amplitude of vibration can be decreased by adjusting the cap 28 to move such electrode away from the reed.

In an organ a number of complete frequency generators will be provided, each similar to the one just described with the exception that the frequencies of vibration of the reeds in each successive generator assembly will be higher by one harmonic. As many harmonics of a basic tone can be provided as may be desirable to build into the instrument, but for practical purposes a series of nine generators will provide the fundamental tone and all harmonics within the audible range of the human ear. Each of these generator assemblies includes its individual control or potentiometer for adjusting the voltage impressed upon the reeds of that generator from zero to maximum so that the timbre of the notes of a keyboard can be composed of a fundamental and various harmonics in the proportions desired, and so that the overall pitch of the keyboardcan be raised an octave by cutting out the fundamental and using a higher harmonic thereof as a fundamental.

It is to be recognized that if the electrode is mounted across from the end of the reed in the manner shown in Fig. 4, the pulsating charge on the electrode has a frequency twice that of the natural frequency of vibration of the reed. This is an important advantage, particularly in higher musical frequencies where very small reeds would otherwise be required. Small reeds require higher air pressure for satisfactory vibration and also naturally have a small amplitude of vibration which would make it difficult to equalize the intensity of the potentials generated by a small reed and a large reed. Therefore, generator assemblies for the higher frequencies can more readily be made to match those producing the lower frequencies if the electrodes thereof are mounted in the manner illustrated by Fig. 4 rather than the mounting described by Figs. 2 and 3.

Key Construction The principal objection to present types of keys from the musical standpoint is that they release a note too mechanically and therefore leave little or no room for the attainment of musical expression. In the key construction now most widely used, movement of the key closes and opens a number of contacts, and from the very nature of an arrangement of that type it is obvious why expression is lacking. The contacts, when closed, instantly release the electrical frequency of the note to the amplifying system and whatever volum is set on the instrument (usually controlled by a foot swell pedal) is simultaneously present. Mechanically, present constructions are costly and require precise workmanship and adjustmen inorder to operate satisfactorily.

For "a note of music to sound properly there should be a gradual initial build-up to themtensity desired and a gradual decay from that level as the note ends, and this criterion obtains for all notes issuing from an instrument but in varying degrees depending upon whether the note is played softly or loudly, staccato or legato. The

ability to release each note with a gradual buildup and decay should be a built-in or inherent feature of an instrument, yet at the same time should be controllable in degree so that the touch and expression of each individual artist become part of the music played, and for the same reason, the volume of each note on a keyboard should be separately controllable by its own key. In other words, each key of the instrument should be capable of completely controlling both the rate of build-up and decay and the volume of its note, and the key construction of the presentinvention is designed to satisfy these criteria as well as to offer many other advantages whi'chwill be more completely discussed herein.

This construction utilizes a transformer for each key as a variable electrical coupling between the frequency generator and the amplifying system, this transformer being provided with a movable secondary coil operated by the key and the movement of this coil being such that the mutual inductance between the primary and secondary coils can be varied from zero to maximum. A construction of this type is shown in Fig. 5 which is a cross-sectional view taken through the key-- board in elevation and including as part of the framework or foundation for the keyboard a front vertical member 30, a horizontal base member 3 I, and a rear vertical member 32. A key 33 is mounted on a pedestal member 34 so as to pivot or rock about the arcuate upper surface 35 thereof and is held in position by engagement between a pin '36 mounted on the member 34, and a V- shaped slot 31 in the key together with the engagement between a second pin 38 and a slot 39 in the key, the pin 38 being carried by the front member 38 on the shoulder 40 of a recess formed therein. I

The key 33 is depressed manually in the usual manner and returned to normal position by a spring 41 secured to the rear vertical member, normal position being defined by a felt stop 42 attached to a member 43 which is only partially shown but which forms part of the cabinet work of the instrument.

A small transformer is located directly beneath the key and consists of two channel-shaped primary core elements 44, a primary winding 45 carried by the inner of these core elements, two secondary core elements 46 and a secondary winding 41 carried by the lower core element. The secondary parts of this transformer are supported by a bracket 43 attached to and depending from the key 33, and including a lower yoke-shaped portion which supports the lower secondary core and the secondary winding 41. This bracket 48 is so designed that when the key 33 is in its normal released position, there will be zero coupling between the primary and secondary of the transformer and movement of the key to full depressed position is such as to cause maximum coupling. As is well known, zero coupling occurs when the secondary coil of a transformer is at right angles to and midway between the ends of the primary coil, while with atransformer of the type here shown maximum coupling takes place when the secondary cores bridge the primary.

Electrically, the primary winding of this transformer is a composite of a plurality of separate windings, each being electrically connected to the electrode of any of the frequency generators which supply the electrical frequency corresponding to the fundamental, together with any of the harmonics thereof which make up the note to be controlled by the key. Thus, if the instrument includes nine frequency generators to provide the fundamental and eight harmonics for certain notes of the keyboard, the primary winding of the transformer of a key for one of those notes will be composed of nine separate superimposed windings as indicated by the nine wires 49 leading from the primary coil 45 in Fig. 5. The secondary coil 41 is conventional and is electrically connected to the input coil of the transformer of the amplifying system as will be later described.

Fig. 6 illustrates in a view similar to that of Fig. one of many possible alternative forms of the construction of a key mechanism. In this construction the key 50 is mounted in the same manner previously described, i. e. for rocking movement about the upper curved surface of a pedestal member (not shown) as constrained by suitable guide pins including afront pin 51 engaging a slot 52 in the key. Return movement occurs under the force of a spring and is limited by a stop such as shown in Fig. 5.

A different type of transformer is used consisting of a channel-shaped primary core 53 supported by a horizontal base member 54. A primary coil 55 is mounted upon one leg of the core 53 and is composed of a number of windings corresponding to the number of frequenciesfundamental and harmonicgenerated for the particular note controlled by the key. The secondary portion of this transformer is mounted for vertical movement with the key 50 and consists of a channel-shaped core 55 and a coil 51 located midway between the horizontal legs of the core 56. Upper and lower brackets 58 and 59, of suitable insulating material, extend from the core 56 and are provided with holes to slidably engage a vertical guide pin 60. A lug 6| forms part of the upper bracket 58, and a link 62 is pivotally attached to this lug and to the key 50 within a recess 63 therein. Fig. 6 illustrates the secondary core and coil in zero coupling position which changes to maximum coupling upon full depression of the key 50 when the lower bracket 59 abuts against a stop 64 and the horizontal legs of the secondary core 56 are aligned with the horizontal legs of the primary core 53.

The advantage of this construction over that of Fig. 5 resides in the fact that zero coupling position is independent of the manner in which the primary coil 55 is wound. Being a composite winding, it becomes difficult in practice to construct this coil so that the number of windings on either side of its physical midpoint are exactly equal, and, therefore, with the arrangement of Fig. 5 there could be some difierence between keys in the relative position of secondary and primary for zero coupling.

With either type of key construction movement of the key from released position varies the relation between the secondary and primary of a transformer from zero or minimum coupling to maximum coupling and thus produces a corresponding variation in the strength of the secondary voltage and thereby in the volume of the note. The exact manner in which this volume is released is directly dependent upon the nature of the movement given to the key by the player, and hence, this type of construction restores 8 what is sometimes referred to as touch to the keyboard enabling each separate note of music to be played as written and permitting each performer to achieve individuality of expression.

Volume control Although the volume of each note is controllable by its key, it is desirable to provide means for varying the overall volume of the keyboard in the manner of the conventional swell pedal used on organs. A representative construction for a well pedal and volume control device employing the variable inductance principle used in the key construction of the invention is shown in Fi 7.

In this view, the primary core and coil 86 of a transformer is mounted upon a horizontal member 81 which forms part of the structure of the instrument. Vertical guide posts 98 extend upwardly from the member 91 at either side of the core 95 and slidably engage a backing member 89 to the under side of which is attached the secondary group of this transformer, consisting of a core 99 and coil 9|. Springs 92 surround each of the guide posts 88 and extend between the member 81 and the backing member 89 to urge the latter upwardly against stop nuts 99 which thus define a position of minimum coupling for the transformer.

The primary coil 85 is substantially rectangular in shape and the secondary coil 9| is formed in the shape of a rectangular shell dimensioned to surround the primary when the core 90 is moved downwardly into juxtaposition with the core 85. Such downward movement towards a position of maximum coupling is in response to conventional downward movement of a swell pedal 94 acting through a suitable linkage. This linkage may, of course, consist in any combination of members which will cause the primary and secondary groups of the transformer to be moved towards a position of maximum coupling upon downward movement of the swell pedal. In the illustration given, the swell pedal 94 is fastened to one arm of a bell crank 95 which is pivoted to a bracket 96 mounted upon a horizontal member 91 of the instrument. The other arm of the bell crank extends through a slot 98 in the member 91 and is pivotally secured to a link 99 which is similarly attached to a second bell crank I09 mounted identically to the bell crank 95. A stirrup-shaped link HH is joined to the bell crank I l and has arms I02 and I03 extending upwardly respectively in front of and in back of the transformer assembly, each of these arms being pivotally secured to the backing member 89.

Depression of the swell pedal 94 thus results in downward movement of the backing member 89 and secondary group of the transformer, increasing the coupling and hence the output of the secondary. When pressure on the pedal is released, the springs 92 return the secondary group to minimum coupling or decreased volume position.

The input transformer of the amplifying system may be used for volume control if desired, in which case the primary winding will be composed of a plurality of individual windings, there being one for each key of the keyboard and the transformer then acts as a variable electrical coupling between the keyboard and the amplifying system. It is to be noted that the volume transformer of Fig. 7 does not have a variation in coupling from zero to maximum, as is the case with the keys, but rather only from minimum to maximum, the minimum level being adjustable to some extent by changing the position of the stop nuts 93 which define the amount of movement of the secondary away from the primary,

' Electrical circuits The electrical relationship between one reed in each of nine frequency generators which provide a fundamental tone and eight harmonics thereof, the key of the keyboard to which the frequencies of such reeds pertain, and the amplifying system of the instrument is schematically shown in Fig. 1. Frequency generators .G-l to G9, inclusive, are each oonnectedto power supply lines 70 and H through potentiometers P! to P-ii in a series of circuits similar to that shown in Fig. 4, so that the voltages passed by each of the potentiometers are impressed upon vibrating reeds R-l to Rr-B, inclusive. The electrodes E-l to 13-9 for these reeds are connected to windings W-l to W-B which together form the primary coil of a key transformer and are associated with the primary core l6 thereof. While these windings are shown for schematic purpose in a series along the core 735, in actual practice they are superimposed. A movable secondary core El and coil 18 are connected to a key l9 and the circuit from the coil 18 leads to the primary coil 80 and core ill of the input transformer of the amplifying system. This transformer is also provided with a movable secondary core 32 and coil 83 for the purpose of controlling the overall volume of the instrument. The remainder of the amplifying system is conventional and hence is not shown in detail.

For the complete instrument the circuit and component parts thereof are duplicated up to and including the primary coil 80 of the input transformer of the amplifying system for each of the designed number of keys of the keyboard.

One of the characteristics of an organ is that it includes means for varying the tone quality or timbre at the will of the player and by tone quality or timbre is means that property which enables one to distinguish between notes of the same pitch and intensity as produced by different instruments. Timbre is determined by the number of overtones or harmonics in a note and by their relative strength or prominence. The timbre of the note controlled by the key 19 can be varied through a great range of tone colors by changing the voltage impressed on the reeds R-l to R4? by means of the potentiometers P-l to P-9. For example, if it is desired to have stopped diapasonic or brilliant reed tone, which is composed of a fundamental and three harmonics, all of the same intensity, potentiometers P-2, P-l, and P45 would be adjusted to deliver equal voltage to reeds R-2, R- i, R-5 and and the remaining potentiometers would be set to give zero voltage to the remaining reeds. With different groups of reeds and different potentials, the complex sound wave required for a note of any timbre can be reproduced.

We claim:

1. A musical instrument of the type described, including in combination means for generating a plurality of electrical currents, electro-acoustic means for translating said currents into sound and key means for regulating the passage of said currents from said generators to said electroacoustic means, said generator means comprising a series of independent electro-static generators,

, each generator having means for creating a plurality of independent electrical frequencies in musical sequence, the range of frequencies of each generator having a harmonic relationship to the range of frequencies of each other generator, a source of electrical energy and independent electrical connections from each of said genera tors thereto, means for independently controlling the current supplied to each generator from said source, key means comprising a series of transformers each having a movable core and a stationary core, a primary winding mounted on one of said cores, a secondary winding mounted on the other of said cores, said primary winding comprising'a plurality of electrically independent separate windings each electrically connected to one of said generators whereby said windings may be independently supplied with electrical currents having frequencies which include a fundamental and various harmonics thereof, and mechanical means connected to said key and said movable core for controlling the position of said movable core relative to said stationary core from a point of zero coupling to a point of maximum mutual inductance between said primary and secondary windings and electrical connections between each of said secondary windings and said electro-acoustic means.

2. The combination set forth in claim 1 wherein said primary winding is associated with said stationary core, the axis of said primary winding is perpendicular to the axis of said second ary winding, and said secondary winding is movable by said key and mechanical means to a position of substantial alignment with the core of said primary winding at said point of zero coupling.

3. In an electrical musical instrument having means for the generation of a plurality of electrical currents having frequencies of musical significance, amplifying means, key means for controlling the passage of a plurality of said currents having a fundamental frequency and various harmonics thereof to said amplifying means, said key means including a transformer having a movable core and a stationary core, a primary winding mounted on said stationary core, said primary winding comprising a plurality of separate windings, each of said separate windings corresponding to one of said fundamental or harmonic frequencies, means connecting each of said windings to the source of one of said fundamental or harmonic frequencies, a secondary winding mounted on said movable core,

and means connecting said movable core to said key whereby the relation of said movable core to said stationary core is controllable between a position of minimum and maximum inductive coupling.

4. An electrical musical instrument comprising in combination a series of generators, each generator having means for creating a plurality of independent electrical frequencies in musical sequence, the range of frequencies of each generator having a harmonic relationship to the range of frequencies of each other generator, independent electrical connections from each of said generators to a source of electrical energy,

= means for independently controlling the currents supplied to each generator, a series of transformers each having a primary and secondary winding, said primary and secondary windings being relatively movable from a position of zero coupling to a position of maximum mutual inductance, one of said windings comprising a plu rality of electrically independent separate windings, connections between each of said separate windings and the source of one of said electrical frequencies in each of said generators whereby isaid windings may be independently supplied with the electrical currents having frequencies which include a fundamental and various harmonics thereof, a key associated with each of said transformers, and mechanical means interposed between said key and its associated transformer whereby the relative position of the primary and secondary Winding of said transformer may be controlled in response to movement of said key from a point of zero coupling to a point of maximum mutual inductance between said windings.

5. The combination set forth in claim 3 wherein said stationary core is a U-shaped member, said primary winding is mounted upon one of the parallel legs of said member and said secondary winding is in substantial alignment with one of said legs at said point of zero coupling.

6. In an electrical musical instrument having means for the generation of a plurality of electrical currents having frequencies of musical significance, amplifying means, key means for controlling the passage of a plurality of said currents having a fundamental frequency and various harmonics thereof to said amplifying means, said key means including a transformer having a stationary winding and a winding movable relative to said stationar winding from a position of minimum to a position of maximum inductive coupling, one of said windings forming a primary winding composed of a plurality of separate windings corresponding in number ta the number of frequencies controllable by said key, means connecting each of said separate windings to the source of one of said fundamen tal or harmonic frequencies, and means for con trolling the position of said movable winding in response to movement of the said key.

7. The invention set forth in claim 6 further including means for varying the intensity of the frequency supplied to each of said separate windings.

PULFORD J. GREAVES. FREDERICK N. ELGEAR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,201,232 Helberger May 21, 1940 2,212,292 Kock Aug. 20, 1940 2,215,709 Miessner Sept. 24, 1940 2,296,125 Traub Sept. 15, 1942 2,297,829 Hammond Oct. 6, 1942 2,302,457 Midgley et al Nov. 17, 1942 2,352,438 Hruby June 27, 1944 FOREIGN PATENTS Number Country Date 478,142 Great Britain June 10, 1938 

